A fault current limiter is a device which can automatically limit fault currents in electrical distribution or transmission grids, in particular low, medium, and high voltage grids, to a low current value close to the nominal current. A benefit of such a device can be that it can reduce drastically the short circuit power of the electricity grid. This can allow interconnect grids without increasing the short circuit power and a decrease in safety margins so that other machineries connected to the network can be designed for lower short circuit power and, therefore, can be made lighter and with a lower effort.
According to known fault current limiters, high-temperature superconductors can be well suited for use in a fault current limiter due to their property to lose superconductivity and transit from the non-resistive superconducting state to a normal state with high electric resistivity when at least one of the critical current, the critical temperature or the critical magnetic field of the superconductor material is exceeded.
In normal operation with nominal current, for example, in the cooled state, the superconductor material is in its superconducting state with essentially zero resistance so that there is essentially no voltage over the whole fault current limiter. In consequence, the fault current limiter is “invisible” for the electricity grid. In case of short circuit fault current, the current can rise to several times the nominal current exceeding the critical current of the superconductor material which causes the superconducting material to transit to the normal resistive state with generation of high voltage. For example, in normal operation in the superconducting state essentially no voltage difference is observed at both ends of the fault current limiter, whereas in fault condition a large voltage difference is measured due to increasing electrical resistance. In order to avoid overheating and damage of the fault current limiter current flow through the fault current limiter should be interrupted within a limited period of time. Thus, a circuit breaker is usually arranged in line with the fault current limiter for switching off the current flow in case of a fault signal detected by fault current detection means.
The document EP 2 242 063 A discloses such a fault current detector for a fault current limiter arrangement. A high-temperature superconductor fault current limiter makes use of a differential protection relay wherein in a fault event the differential protection relay operates a triggering mechanism of a circuit breaker, whereby opening the electrical circuit and interrupting power supply to downstream components. The current limiter arrangement includes a current incoming end and a current outgoing end. The incoming end of the fault current limiter is electrically connected to a first measuring circuit and the outgoing end of the fault current limiter to a second current measuring circuit. In the fault event, voltage drop occurs across the current limiter with the voltage at the outgoing end of the current limiter being less than the voltage at the incoming end. Consequently, a current difference can be observed by the differential protection relay. If the observed value of the current difference exceeds the predetermined value, the differential protection relay sends a triggering signal to the circuit breaker and current flow is interrupted.
For operating the fault current limiter liquid nitrogen can be used in order to maintain the super-conductance property. To provide the cooling, permanent losses of energy can occur over the years.